The invention concerns a cryostat comprising a vacuum container which houses a chamber with at least one object to be cooled, wherein the vacuum container has at least one hollow neck tube which connects the chamber through the outer shell of the vacuum container to the area outside of the cryostat, wherein the neck tube houses a cooling arm of a cold head, wherein the cooling arm is thermally connected to a refrigeration device and can also be brought into thermal contact with a second thermal contact surface on the object to be cooled via a first thermal contact surface on the cooling arm.
A cryostat of this type is disclosed e.g. in U.S. Pat. No. 5,934,082 or U.S. Pat. No. 4,535,595.
In most cases, cryotechnology utilizes cooling machines for cooling objects, e.g. superconducting magnet coils. The cooling machines discharge heat from the apparatus containing the object to be cooled by means of a cold head.
These cooling machines are typically operated with helium gas as the coolant which is compressed in a compressor and expands in the cold head of the cryostat (e.g. so-called “pulse tube coolers”). The cold head and the compressor are generally connected to each other via two pressure lines. The cold head is connected to the components to be cooled either directly mechanically or via a contact medium (e.g. cryo gas or cryo liquid) or in both ways in order to ensure good heat transfer.
However, if, e.g. due to a technical defect or power failure, the compressor fails completely or partially, the previously cooled components are heated. In this situation, the cold head of the cryostat then represents a substantial thermal bridge between the components to be cooled and the external surroundings.
In its persistent operating mode, the superconducting current in a superconducting magnet can flow practically without resistance for extremely long time periods. However, heating of the magnet causes a so-called quench of the persistent operating mode after a certain time. At some point, the magnet reaches the critical transition temperature which is predetermined by the superconducting material and becomes normally conducting and thereby loses, generally abruptly, its high magnetic field.
A reduction of the thermal load after failure of the cooling machine would at least considerably extend the time period until a quench happens. This is true, in particular, for cryostat configurations that can be operated completely without or merely with minimum amounts of liquid coolant, wherein superconducting magnets are currently normally operated in a liquid helium bath.
U.S. Pat. No. 6,164,077 discloses replacement of the thermal contact between the cooling arm and the object being cooled with gas in the event of failure of the cooling device.
Since helium is becoming more and more expensive, cryostats that can be operated completely without or at least with minimum amounts of helium (low-loss or even cryo-free systems) are becoming more and more attractive both technically and economically.
However, the thermal capacity of solids significantly decreases at very low temperatures. For this reason, it would be particularly important for systems of this type using little amounts of liquid helium or no liquid helium at all to minimize the heat input into the object to be cooled in case of failure of the cooling unit.
U.S. Pat. No. 7,287,387 B2 describes a cooling unit for cooling a superconducting magnet coil and the radiation shields or chambers that surround it. Whereas cooling of the radiation shields or chambers is effected via direct thermal contact, the coil is cooled by means of re-liquefied helium. Bellows are used at the interface between the housing and the cooling unit in order to obtain vibrational decoupling. The cooling unit always remains in fixed contact with the radiation shield and the inner chamber. A pressure change in the inside of the cryostat does not change the thermal contact. It is only stated that the bellows should withstand an overpressure of 1 bar.
U.S. Pat. No. 8,069,675 B2 also describes a cold head that is flexibly connected to the cryostat. In this case, however, an actuator is operated in order to release the thermal contact. It is not an automatically functioning passive system but requires active intervention by an operator. The same also applies for the cooling configurations as disclosed e.g. in U.S. Pat. No. 5,522,226 or U.S. Pat. No. 5,430,423.
EP 0 366 818 A1 discloses a configuration with which the adjustment of the penetration depth of a cold head into a LN bath is done automatically in dependence on the pressure within the cryostat.
The above-cited U.S. Pat. No. 5,934,082 discloses a “cryo-free system”, wherein the cold head is in thermally conducting physical contact both with a heat shield and a magnet coil. The hollow space between the heat shield and the cold head is evacuated in this connection. Spring elements are provided in the cooling device for absorbing or damping oscillations.
U.S. Pat. No. 4,535,595 also describes a similar cooling system. Also in this case, the gas is not in direct contact with the cold head but the hollow space is again evacuated. This document moreover discloses a cold head that can be displaced in a vertical direction and is also in thermal contact with a heat shield and a magnet coil.
In contrast thereto, it is the underlying object of the invention, which is relatively demanding and complex when regarded in detail, to significantly and operationally safely reduce the thermal load by the cold head onto the object to be cooled in case of failure of the cooling machine in a cryostat of the above-mentioned type with simple technical means and fully automatically without requiring the intervention of an operator, wherein already existing devices can be retrofitted with as simple means as possible.